The present invention relates generally to wood-based composite boards or panels such as particle board, oriented strand board, waferboard, fiberboard and the like. More particularly, the present invention relates to a method and the composite board product made from using materials having high thermal conductivity, such as carbons, metals, carbides and nitrides, as fillers in the manufacturing of such particle boards.
Wood-based composite board is typically a panel manufactured from wood materials, primarily in the form of particles (particleboard), flakes (oriented strand board (OSB) or waferboard (random-orientation of flakes)), and fibers (medium density fiberboard, MDF), combined with a thermoset resin and bonded at an elevated temperature and an elevated pressure typically in a hot press. As a result, the process is sometimes referred to as hot pressing. In such a manufacturing process, the productivity or throughput of a plant or a production line depends heavily on the major production steps including wood drying, resin application, and hot pressing. Hot pressing is considered a costly unit operation. A reduced pressing time, therefore, will have positive impacts on lowering the production costs and increasing wood board output capacity. Hence, the wood-based composite industry is constantly looking for strategies to reduce the pressing time.
The pressing time, which is defined as the time it takes to compress a mat made of wood pieces and resins to the final board thickness once the press platens make contact with the mat surfaces, must be sufficiently long to allow proper curing of the thermoset resin present in the mat. The heat transfer from the mat faces to the core is a critical factor in determining the curing rate of the resin, thus the final press time. Because of the low heat conductivity of wood and wood pieces, the cure of the resin takes place first in the faces when the hot platens are in contact with the mat during pressing while the core of the board is still cold. Consequently, a fairly long time will be needed for sufficient heat to reach the core of the board, which will allow the middle (core) of the board to cure.
In conventional approaches to solving the problem, high press temperatures and/or an external catalyst is often used to accelerate the resin cure rate to obtain acceptable press times. Usually a dilute acid or an acidic salt (ammonium sulfate and chloride) in a 20 to 30% aqueous solution is used as external catalyst. Sodium hydroxide, carbonates, and other similar basic compounds also may be used to promote fast cure of the resins by accelerating the polymerization of certain thermoset resins. The external catalyst, however, is generally acceptable only if it will produce shorter press times at a low concentration in the range of 0.25 to 1% by weight of the resin.
Higher pressing temperatures generally are not preferred partly due to higher energy consumption. More importantly, higher press temperatures tend to generate more volatile organic compounds (VOCs) that may adversely affect the environment. Additional equipment or steps may be needed to prevent escape of volatile organic compounds to the atmosphere. Although press times may be shortened through these approaches, there is still a need to develop more cost-effective and environmentally friendly approaches to reduce the press times.
Water absorption and dimensional stability of the wood composition board are additional important property issues faced by the industry. Because wood absorbs water, the thickness of the panel swells and the panels decay when they come in contact with water or moisture. Since most of the thickness swelling is not reversible when the panels are re-dried, the products are regarded as undesirable or unacceptable for many applications (such as siding) where high moisture situations are expected. It is therefore desirable to reduce water absorption and increase dimensional stability.
The thickness swelling of wood-based composite panels depends on both the nature of their constituents and the manufacturing process. As already discussed, the panels are made up of small pieces of wood bonded together with adhesives or thermoset resins at elevated temperature and elevated pressure to develop adequate mechanical strength properties. During this process, the wood is typically densified by a factor up to 1.3 times higher than its original density. Internal stresses are induced and built-up within and between flakes/particles during densification. When the panel absorbs water, these internal stresses are relieved and the compressed wood springs back to regain its natural form and density. As a result, the panel swells in the thickness direction.
Extensive research has been conducted to reduce water absorption and thickness swelling of wood-based composite panels. It has been shown by a number of investigators that minimizing the compressive stress during and/or after panel consolidation can significantly reduce the spring back phenomenon, and a dimensionally more stable panel can be produced. Many methods have been suggested to control thickness swelling of wood-based composite panels. These include the use of high adhesive (typically thermoset resins, also called binder) content, longer press time and/or much higher press temperature during panel manufacturing. The use of a steam-injected press and the post-press treatment of the board with steam in a pressure vessel also have been suggested. Addition of emulsion wax (0.25-1% wt of oven dried wood) to the wood furnish during resin application has been reported to lower the water absorption of the panel. The cost-effectiveness of all these approaches remains unclear.
The present invention relates to a wood composite for making board or panel and the like comprising a plurality of wood pieces; a thermoset resin capable of binding the wood pieces; and a filler having a high thermal conductivity. The wood pieces are in a form selected from particles, flakes, fibers or mixtures thereof. The wood pieces or the product board (or panel) also may be acetylated. The filler may be selected from a group consisting of metals such as aluminum, iron, tungsten, zinc, copper, tin, titanium and mixtures thereof; carbon filler such as natural graphite, synthetic graphite, scrap graphite, carbon black, carbon fiber, metal (such as nickel) coated carbon fiber, carbon nanotubes, coke and mixtures thereof; a nitride such as silicon nitride, carbon nitride, boron nitride; a carbide such as silicon carbide; conducting polymers; and mixtures thereof. The thermoset resin is selected from the group consisting of phenolic resin, MDI resin, urea resin, melamine resin, epoxy resin, urethane resin, particularly non-foaming urethane resins and mixtures thereof. A catalyst may also be added to the composition to accelerate curing of the thermoset resin and/or reducing the hot-pressing time.
The present invention also relates to a method for manufacturing wood composite board (including panels), the method comprises mixing a thermoset resin and a plurality of wood pieces to form a blend; adding a filler having a high thermal conductivity to the blend to form a mixture; placing the mixture in a shaped container; and applying an elevated temperature and an elevated pressure to the mixture in the shaped container to form the wood composite board.